Experiments are proposed to investigate the role of photoperiod and the pineal gland as regulators of reproduction in the male laboratory rat. The aim is to further elucidate a unique model system: induction of seasonal-like reproductive responses in a classic, nonseasonal breeder. Studies will probe mechanisms which underlie the ability to stimulate gonadal regression and attendant effects in the male laboratory rat. Two focal points include: fundamental experiments to establish both the validity of the model and a framework from which it can be compared and contrasted to responses of some seasonal species; and, secondly, the neural basis for the response. Since testicular regression in the laboratory rat model system depends upon changes in lighting, the pineal gland and testosterone administration, proposed experiments will assess mediation by these factors and the neuroendocrine system. Specific aims will address: whether a complete annual reproductive cycle exists in the induced "photoperiodic" rat and the degree of interaction and control among the pineal gland, photoperiod and testosterone in regulating testicular regrowth following regression; and an evaluation of the pineal gland's role as an interface between the circadian timing system and the hypothalamic-pituitary-gonadal axis. Experiments will measure the rhythm of the pineal gland hormone, melatonin via radioimmunoassay following changes in photoperiod and after different exposure periods, in an attempt to identify what characteristic(s) of the melatonin signal effects reproductive collapse. A pilot study is designed to probe the response of the "photoperiodic" rat to natural photoenvironments within the laboratory regarding activity cycles and seasonal testicular regression. This research will increase our understanding of the environmental control of reproduction. Since some or all aspects of a seasonal reproductive response remain extant in the common lab rat and can be elicited under certain conditions, this may be another exciting example of neural plasticity at the level of physiological timekeeping. These studies seek to identify the limits of the response in the rat and the specific neuroendocrine changes which underlie the evolution of a nonseasonal breeder. Additionally, since it is possible to express a significant regulatory role for the pineal gland in the nonseasonal laboratory rat, the proposed studies provide a framework potentially applicable to a role for the pineal gland in the economy of human reproduction.